Bio-inspired actuation materials, also called artificial muscles, have attracted great attention in recent decades for their potential application in intelligent robots, biomedical devices, and micro-electro-mechanical systems. In particular, ionic-polymer metal-composites (IPMCs) as actuators are positioned as promising candidates due to their bending actuation at ultralow voltages and are among the most promising electroactive polymer (EAP) materials for artificial muscle constructed with ionic-conductive electrolytes, mobile molten ionic salts, and sandwiched metallic conductors. A typical electromechanical actuator is composed of one ion-conductive electrolyte membrane laminated by two electron-conductive polymer electrode membranes, which can bend reversibly due to the electrode expansion and contraction induced by ion motion under alternating applied voltage. As its actuation performance is mainly dominated by electrochemical and electromechanical processes, the electrode, electrolyte material, and structure play an important role to attain high-performance actuation. The recent discovery of ionic two-dimensional covalent-organic frameworks (ICOFs) has created a revolution in functional nanomaterials. Their unique ordered structures render them intriguing ionic-conductivity properties, which make them ideal electrolyte materials for electrochemical actuators. The presented outstanding actuation performance gives us tremendous motivation to tackle the challenges in understanding the mechanism and developing more advanced actuators. The present study ascertains the functionality of soft electrolytes for bionic artificial actuators while providing ideas for expanding the limits in applications for soft robots.

    Recently, the research groups of Prof. Pooi See Lee reported a work entitled “Ionic Covalent Organic Framework based Electrolyte for Fast-Response Ultra-Low Voltage Electrochemical Actuators” in Nature Communications (DOI: 10.1038/s41467-022-28023-2). For the first time, an electrochemical actuator based on the oriented ionic two-dimensional (2D) crystalline COF was demonstrated. The actuation performance was enabled and improved through the ordered pore structure of opening up efficient ion transport routes (see the following figure). The ordered ionic COF electrolyte was sandwiched between two PEDOT:PSS conductive polymer electrodes through a casting method and the actuation performances of the as-prepared films have been comprehensively investigated. It was found that the actuator demonstrates high actuation performance, with large actuation displacement (~9.6 mm), a large strain difference of 0.39%, a rapidly attained equilibrium-bending motion (~1 s), a broad-band frequency response of 0.1−20 Hz, and long-term durability in the air (>23,000 cycles) under ultra-low electric stimulus (≤ ±0.5 V). Such actuation performance is due to its ordered pores (~2.4 nm), large specific surface area (381 m² g^-1), and high ionic conductivity (13.5 mS cm⁻¹). The regular interlayer accumulation and well-order orientation of ionic COF electrolyte structure offers efficient transmission and extensive diffusion channels to facilitate mass transportation and ion intercalation/de-intercalation of solid electrical elements in electrodes.

    Our research confirms the importance of crystallinity and order two-dimensional structure in developing external-stimulus-responsive COF materials, which would provide some guidelines in improving the performances of COFs through the design of novel building units. We anticipate that the method described in this study will open up possibilities for the efficient development of high-performance actuator materials by providing a basis for multifunctional actuators.

To read more about our work at Nature Communications:

https://doi.org/10.1038/s41467-022-28023-2